1. Field of the Invention
The present invention relates to a drawing instrument, and more particularly to a drawing instrument suitable for drafting angular lines.
2. Description of the Prior Art
A typical drawing instrument is disclosed in U.K. Patent No. 738,223 and comprises an arm 2 having a circular portion 5 formed on one end thereof and rotatably engaged in a base 1. In this drawing instrument, the arm 2 is apt to be disengaged from the base 1, particularly during drafting operations, such that the user may feel inconvenient. In addition, when drafting two sides of an angle with the arm 2 and the top edge 4 of the base 1, the coupling portion of the sides of the angle will be obstructed by the circular portion 5, such that the drawing instrument should be removed before the angle can be completed. This is also inconvenient.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the contentional drawing instruments.
Another drawing instrument is disclosed in U.S. Pat. No. 715,194 to Callahan, issued Dec. 2, 1902, entitled "COMBINED RULE AND SQUARE". Such a ruler includes two rulers each having a head portion slidably coupled together such that the rulers are rotatable relative to each other. However, one of the head portions includes a single hinge part which is rotatably engaged between a pair of parallel hinge parts formed on the other head portion. Since the rulers are made thin, such that the pair of hinge parts should be made thinner and such that the hinge parts are apt to be broken. In addition, a pin should be engaged between the center of the pair of hinge parts and another pin is required to be provided in the radially outward portion of the pair of hinge parts for sliding engagement in a curved slot formed in the single hinge part; it is difficult to form the pins between the pair of hinge parts such that the manufacturing processes are complicated.
Another type of drawing instrument is shown in U.K. patent No. 1,134,676 to Bseso. Similarly, a bolt and nut is required for coupling the rulers together, in addition, the rotational angles of the rulers are limited such that the rulers may not rotate relative to each other for 180 degrees.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional drawing instruments.